Fall Fertilizer Applications for the 2021 Crop

The 2020 growing season has come to an end or the end is in sight. Shortly, decisions will be made on a fertilizer program for the 2021 crop year. The importance of having an up to date soil sample cannot be stressed enough. At a minimum, a composite soil sample should be collected on each field. Ideally, some sort of precision soil sampling should be done to identify spatial variability and to more clearly determine soil fertility levels across the entire field. In the current economic climate, it is more important than ever to make sure the right nutrient is being applied in the right quantity to ensure higher yields. Making fertilizer decisions based on historical applications rather than actual data opens the door to poor decisions.

South Texas has a great deal of variability in soil types and inherent fertility levels. The importance of nitrogen and phosphorus applications has been stressed historically. Both nitrogen and phosphorus are extremely important but ignoring other nutrients can significantly reduce yield potential. The top two most deficient nutrients in South Texas soil tests are phosphorus and zinc. However, potassium and sulfur are not very far behind. In the current agricultural climate, it is very important to prioritize fertility needs based on the goal of achieving a return on investment. If soil tests from the farm identify phosphorus, zinc, potassium, and sulfur as the most deficient nutrients but the pocketbook dictates that only two of nutrients can be addressed, how should that decision be made?

Focusing on the historical issue of low phosphorus may not always give the largest return on investment. In cotton specifically, potassium and zinc are the top two deficient nutrients in plant tissue samples collected at early and mid-bloom. At early bloom, potassium and zinc deficiency in the plant are ~3.5 times more prevalent than phosphorus deficiency. At mid-bloom, potassium and zinc deficiency in the plant are ~9 times more prevalent than phosphorus deficiency. This does not necessarily mean there needs to be a drastic increase in fertilizer spending or any increase at all. It does indicate the potential to shift some fertilizer dollars from phosphorus to potassium and zinc.

With relatively low and static commodity prices that bring along low margins, it makes sense to revisit how fertilizer dollars are being spent. By attaining an up to date soil sample and looking at your own or regional plant tissue samples, there may be an opportunity to make some changes. In many cases, small adjustments can bring positive results. This can be as simple as reducing phosphorus spending 15-25% and using those dollars for an additional foliar application of potassium and zinc in season. In some situations, there is an opportunity to reduce larger amounts of phosphorus and replace it with potassium fertilizer. The key is that there is no silver bullet and every grower, farm, or field may be in different situation. Hence the importance of having up to date soil samples and access to plant tissue sample data to aid in the fertilizer decision process.